Audio compression is commonly employed in modern consumer devices for storing or transmitting digital audio signals. Consumer devices may be telecommunication devices, video devices, audio players, radio devices and other consumer devices. High compression ratios enable better storage capacity, or more efficient transmission via a communication channel, i.e. a wireless communication channel, or a wired communication channel. However, simultaneously to the compression ratio, the quality of the compressed signal should be maintained at a high level. The target of audio coding is generally to maximize the audio quality in relation to the given compression ratio, i.e. the bit rate.
Numerous audio coding techniques have been developed during the past decades. Advanced audio coding systems utilize effectively the properties of the human ear. The main idea is that the coding noise can be placed in the areas of the signal where it least affects the perceptual quality, so that the data rate can be reduced without introducing audible distortion. Therefore, theories of psychoacoustics are an important part of modern audio coding.
In known audio encoders, the input signal is divided into a limited number of sub-bands. Each of the sub-band signals can be quantized. From the theory of psychoacoustics it is known that the highest frequencies in the spectrum are perceptually less important than the low frequencies. This can be considered to some extent in the coder by allocating lesser bits to the quantization of the high frequency sub-bands than to the low frequency sub-bands.
More sophisticated audio coding utilizes the fact that in most cases, there are large dependencies between the low frequency regions and high frequency regions of an audio signal, i.e. the higher half of the spectrum is generally quite similar as the lower half. The low frequency region can be considered the lower half of the audio spectrum, and the high frequency can be considered the upper half of the audio spectrum. It is to be understood, that the border between low and high frequency is not fixed, but may lie in between 2 kHz and 15 kHz, and even beyond these borders.
A current approach for coding the high frequency region is known as spectral-band-replication (SBR). This technique is described in M. Dietz, L. Liljeryd, K. Kjörling and O. Kunz, “Spectral Band Replication, a novel approach in audio coding,” in 112th AES Convention, Munich, Germany, May, 2002 and P. Ekstrand, “Bandwidth extension of audio signals by spectral band replication,” in 1st IEEE Benelux Workshop on Model Based Processing and Coding of Audio, Leuven, Belgium, November 2002. The described method can be applied in ordinary audio coders, such as, for example AAC or MPEG-1 Layer III (MP3) coders, and many other state-of-the-art coders.
The drawback of the method according to the art is that the mere transposition of low frequency bands to high frequency bands may lead to dissimilarities between the original high frequencies and their reconstruction utilizing the transposed low frequencies. Another drawback is that noise and sinusoids need to be added to the frequency spectrum according to known methods.
Therefore, it is an object of the application to provide an improved audio coding technique. It is a further object of the application to provide a coding technique representing the input signal more correctly with reasonably low bit rates.